In general ionising radiation is considered to be radiation within the energy range 5 KeV to 6 MeV and includes gamma rays, x-rays, beta-rays, alpha-rays and neutron beams. Devices for detecting ionising radiation are well-known for radiological protection and metrology, such as in health or nuclear physics as well as national/homeland security and anti-terrorist applications. The devices are one of two types, either passive detectors or electronic-based active detectors.
Passive detection systems use film (film-badges), thermo-luminescent detection (TLD) or photochromatic technologies (PC) as detector materials. Common to these detector technologies is that they register the presence of ionising radiation by a change of state. For example, a film exposed to ionising radiation goes dark when developed, TLD materials emit light when heated having previously been exposed to ionising radiation and PC materials change colour when irradiated with ionising radiation. However, the change of state of these materials requires special processing in order to be determined, for example developing the film or heating the TLD material. Consequently, only an historic monitoring and evaluation of radiation exposure can be obtained. It is not possible to achieve real-time monitoring and evaluation. Since no direct real-time monitoring or analysis is possible it is therefore necessary to infer what type of radiation exposure caused the change of state. Although such inference can be drawn based on experience, nevertheless it is not possible to precisely determine what type of radiation (spectroscopic information) has been sensed nor an estimate of radiation dose which takes into account such information. Additionally, known passive detection systems generally have poor sensitivity to ionising radiation.
Active detectors may be based upon silicon technology and generally comprise one, two or three PIN-diodes, each PIN-diode having a preset threshold level to signal an alarm relating to a minimum energy level of incident radiation. If more than one PIN-diode is used then different threshold levels may be preset corresponding to different radiation and energy levels thereby providing crude spectroscopic analysis of incident radiation. However, silicon has poor sensitivity to ionising radiation since it does not have a high atomic number (Z), therefore there is inefficient conversion of the incident radiation to electric current and devices using such technology suffer from poor signal to noise ratio.
Higher Z materials may be used for the diode, such as mercury iodide (MgI) or lead iodide (PbI), which have better radiation conversion efficiency. Optionally, high Z scintillator materials such as sodium iodide (NaI) and caesium iodide (CsI) may be used which are normally formulated of single large crystals. To overcome poor sensitivity, many ionising radiation detectors use a large area (often single detector crystals) of detector material, however this results in saturation of the detector material when irradiated by a high radiation flux.
Detectors using scintillator material are generally bulky. This is due to the fact that the mean free path (the average distance traveled by a photon before interaction) of medium energy photons in such materials results in the use of crystals of a few cubic cm rather than millimeters. Additionally space needs to be reserved for the photo-multiplier tubes or photo-diodes required to convert the light pulses to an electronic signal adequate for the subsequent processing circuitry. Another drawback with using scintillator detectors is that light is radiated in all directions and therefore much of the energy resulting from the conversion of the incident radiation to light in the crystal does not reach the photo-multiplier tube and detector circuitry, hence decreasing the signal to noise ratio of the final signal.
Use of semi-conductors with higher Z than silicon for detecting ionising radiation, for example cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) is already known, for example in the Amptek Inc. detector XR-100T-CdTe. Generally, such detectors utilise a large slab (for example around 3 mm by 3 mm by 1 mm) of detector material with very fast read-out. However, such a large area, although sensitive, also limits the radiation flux intensities which can be detected since a large area detector has a tendency to saturate at high flux intensity. Such single crystal slab large area detectors rely on fast read-out, for example using a multi-channel analyser, to ensure that the signals transformed into single counted events result from single photo-electric interactions.
Typically wire bonding is used to electrically couple the detector substrate to the detector electronics. Such wires by virtual of the mechanical requirements of the connection are normally characterised by large capacitance values which mitigates against fast read-out, and also acts as an antenna and picks up spurious electrical signals thereby degrading the resultant signal to noise ratio. Additionally, the wire connections inherently introduce losses which limits the sensitivity of the detector.
Another drawback of known active detectors is that the electronic signals are generated remote from the detector substrate, leading to signal losses and signal mis-shaping due to the impedance of connecting wires and circuitry.
The present invention was devised with the foregoing in mind.